Various types of implants are commonly used in biomedical applications, particularly in the dental and orthopedic fields. Often, implants are associated with use in areas of hard tissue (i.e., cartilage, bone, etc.), and the implants generally comprise hard, durable materials, such as metals, particularly titanium.
Uncoated titanium implants are normally covered by a bioinert surface of titanium dioxide. The presence of the bioinert surface structure prohibits biointegration of the implant by the surrounding tissue. Accordingly, the body responds to the foreign object by isolating the implant with a flexible layer of fibrous tissue that can easily cause an implant to loosen. This is detrimental to the usefulness of the implant. For example, in the case of dental implants, loosening of the implant can result in loss of the implanted tooth and can also lead to infections around the loosened implant.
It is commonly known in the art to apply various coatings to orthopedic components and other medical devices for a variety of reasons, including facilitating implant fixation and bone in-growth. See, Handbook of Materials for Medical Devices, Davis, J. R. (Ed.), Chapter 9, “Coatings”, (2003). In particular, calcium phosphate phases are useful as coatings for facilitating bone in-growth. One calcium phosphate phase, hydroxyapatite (HA) [Ca10(PO4)6(OH)2], is the primary mineral content of bone and calcified cartilage, representing 43% by weight of bone. Because of the chemical and crystallographic similarities with the inorganic components of bone, applying a thin layer of HA, or other calcium phosphate layer, to the surface of a metal implant, such as a titanium implant, can promote osseointegration and increase the mechanical stability of the implant. In fact, many studies have demonstrated that dental and orthopedic implants coated with plasma sprayed HA promote greater direct bone attachment and higher interfacial strength compared to the uncoated titanium implants. Numerous problems with the HA coatings, however, have also been cited, including variation in bond strength at the coating-metal interface, variation in structural and chemical properties, and non-uniformity in coating density.
Hydroxyapatite coatings are generally comprised of varying percentages of crystalline HA, tricalcium phosphate, and amorphous calcium phosphate. The ratio of HA to tricalcium phosphate has been reported to be crucial for bone regeneration. It has also been reported that the dissolution rate of a HA coating is correlated to the biochemical calcium phosphate phase of the coating. It is known that coatings with more crystalline HA are more resistant to dissolution. Conversely, coatings with increased concentrations of amorphous calcium phosphate and tricalcium phosphate are thought to predispose the HA coatings to dissolution. Since it has been suggested that the dissolution of calcium phosphate from the surface of the implant in the body is responsible for the bioactivity of the HA coating, knowledge of the crystalline content of the surface coating is critical to implant success. Some studies have indicated that bone responds differently to HA coatings of different crystallinity. These studies have indicated higher bone activity with well characterized HA coatings of higher crystallinity, while other studies suggest that some amorphous phase in the coatings is desirable and promotes a more stable interface with the biological environment. Still further studies have identified various structural factors that also affect the biological response of bone to HA coatings, including surface texture, porosity, and the presence of trace elements. Accordingly, it is beneficial for the characteristics of the implant surface to be precisely controlled during the implant process, particularly with respect to the crystalline content of the coating surface.
Traditional HA coatings are deposited by various techniques, such as sputtering, electron beam deposition, laser deposition, and plasma spraying. Because of its simplicity and versatility, plasma spraying is the most widely used technique. Although plasma spraying is fast and cost effective, the coatings have several flaws that could lead to implant failures. Plasma sprayed films exhibit a high porosity and only attach to the substrate surface through mechanical bonding (i.e., no intermolecular bonding). This leads to inconsistent bonding strengths. Further, regardless of the coating methodology, amorphous layers are generally formed on metal substrates, which have a high dissolution rate in aqueous solutions. Therefore, the layers are often subsequently heat-treated at approximately 600° C. to convert the amorphous phase into a crystalline phase. This heat treatment, however, causes cracks in the layer due to the thermal expansion mismatch between the coated layer and the metal substrate. This leads to a severe reduction in bond strength.
Plasma sprayed coatings are also relatively thick. Generally, coatings on commercially available plasma sprayed implants have a thickness of between 79 μm and 111 μm. Such thick coatings can lead to low fracture resistance. This, along with reduced bond strength, can lead to delamination, and detached fragments have very adverse effects on the implant, as well as the tissue surrounding it. For example, particulate debris at the bone prostheses interface with HA coated implants has been found to cause a foreign body response that is destructive to the surrounding tissues. As a result, improvement of the HA coating properties may reduce shedding and possibly prevent an aggressive osteolytic response. Some studies have indicated that thin HA coatings (about 2 μm) have a significantly greater coating-metal interfacial strength compared to commercially available thick (70 μm) plasma sprayed HA coatings (40 MPa versus 9 MPa, respectively).